It is often desirable to measure the value of a real world variable. The real world variable may be temperature, pressure, voltage, current flow and the like. In many situations a measurement system provided to measure the real world variable may transform the real world variable into another variable, which is typically an electrical signal. For example, when measuring a voltage where that voltage may be a relatively high voltage, it is often desirable to pass the voltage through a potential divider in order to transform it into a voltage range that is convenient for subsequent processing, for example by an analog to digital converter. Thus, the potential divider has a transfer function, both in terms of its attenuation at direct current DC (0 Hz), and also how the transfer function may vary with frequency. Similarly, if current is to be measured it may be measured by use of a shunt, or a current transformer, amongst other current measuring devices. Again the shunt, transformer or so on has a transfer characteristic such that the input current is transformed to an output value. That output value may then, once again, be processed by subsequent processing circuitry. Other components in the signal path, such as variable gain amplifiers and analog to digital converters also affect the signal, and the whole of the signal processing chain from the sensor to the analog to digital converter can be represented by a combined transfer function.
The provision of a transducer, circuit or other component which exhibits a transfer function means that the output signal is now a function of the unknown input signal, which can be regarded as a “measurand” as it represents a quantity which is to be measured, and the transfer function of the measurement circuit. Thus greater uncertainty has been introduced into the output result.